hooks: define hooks for startup and shutdown

hooks: define hooks for startup and shutdown

Having hooks which run when mutt first starts (startup-hook) but before the
fist mbox is opened, and when mutt closes the mbox (shutdown-hook) before
exiting, allows for those notmuch users to run commands to sync the state of
their caches, etc.

support GLOBALHOOK type

There's now a few hooks (startup/shutdown/timeout) which don't operate on a
mbox or pattern, but are instead global to mutt.  Therefore, don't require the
hooks themselves to needlessly define these parameters.

Closes: #184